1. Field of the Invention
The present invention generally relates to an organic electro-luminescent (EL) device (also known as OELD) and a method for fabricating the same, and more particularly to a white light emitting organic EL device able to directly emit a continuous full-color light providing three different frequency bands and a method for fabricating such an organic electro-luminescent device.
2. Description of the Prior Art
The organic electro-luminescent device has attracted tremendous attention due to its advantages over other display panels. These advantages include self-luminescence, large visual angle, short response time, compact size, light weight, reduced dimension in thickness, high brightness, low power consumption, simple fabrication, and the ability for light emitting in a full-color range. Therefore, such an organic electro-luminescent device is increasingly required to replace the currently used white light sources such as fluorescent lamps and light bulbs to save energy, and the technologies thereof have widely been studied in the industry all over the world.
Please refer to FIG. 1, which is a cross-sectional view showing the structure of a conventional organic EL device in accordance with the prior art. The organic EL device is characterized in that a bottom electrode 13 is formed on a substrate 11, and on the bottom electrode 13 there are formed an organic layer 15 comprising a hole injection layer, a hole transport layer, an organic emitting layer or an electron transport layer, an opposed electrode 17 and an sealing cap layer 19 by evaporation in turn. Due to the limitation of the material used in the organic layer 15, such a device as described in FIG. 1 is only able to emit monochromatic lights, preferably blue, red, or green according to various chosen fluorescent materials, and fail to achieve the object of emitting a white light or continuous full-color light. Therefore, several modifications have to be made on the device structure to obtain a white light source.
For example, Forrest et al. (The Trustees of Princeton University, Princeton, N.J.) discloses xe2x80x9cMulticolor organic light emitting devicesxe2x80x9d as U.S. Pat. No. 5,757,026 and Chao, et al. (Industrial Technology Research Institute, Hsinchu, TW) discloses xe2x80x9cMethod for fabricating an organic electro-luminescent devicexe2x80x9d as U.S. Pat. No. 6,037,190. Please refer to FIG. 2, which is a cross-sectional view showing the structure of a conventional white light emitting organic EL device similar to the organic EL device disclosed in U.S. Pat. No. 6,037,190. In FIG. 2, the white light emitting organic EL device is characterized in that three organic layers 25G, 25R and 25B for emitting a green light, a red light and a blue light, respectively, are interposed between the bottom electrode 13 and a corresponding opposed electrode 17. Therefore, a white light source is obtained by mixing the lights from the three organic layers.
Nevertheless, there exist some problems in the aforementioned white light emitting organic EL device, in that:
(1) the organic material for emitting a red light has a shorter lifetime than other organic materials, which makes the white light emitting organic EL device mal-functional because the organic material for emitting a red light has failed;
(2) the manufacturing processing for forming different organic layers on a chip is difficult to carry out; and
(3) hardware,implementation is difficult because the voltages for activating different organic layers are different.
In order to avoid the problems resulting from the organic EL device in FIG. 2, there have been proposed a number of disclosures such as U.S. Pat. No. 6,008,578 xe2x80x9cFull-color organic electro-luminescent device with spaced apart fluorescent areasxe2x80x9d filed by Chen (Hsinchu, TW), U.S. Pat. No. 5,294,870 xe2x80x9cOrgahic electro-luminescent multicolor image display devicexe2x80x9d filed by Tang, et al. (Eastman Kodak Company, Rochester, N.Y.) and U.S. Pat. No. 5,717,289 xe2x80x9cThin film electro-luminescent element easily regulating emitted light to whitexe2x80x9d filed by Tanaka (NEC Corporation, Tokyo, JP). Please refer to FIG. 3, which is a cross-sectional view showing the structure of a conventional white light emitting organic EL device similar to the thin film electro-luminescent element disclosed in U.S. Pat. No. 5,717,289. In FIG. 3, the white light emitting organic EL device is characterized in that a color conversion layer 30 is interposed between the bottom electrode 13 and the substrate 11. The color conversion layer 30 is formed of a fluorescent material. When a current is injected from the bottom electrode 13 into the opposed electrode 17, the organic layer 15 emits a blue light (B). As the blue light (B) passes through the color conversion layer 30, a certain percentage of the blue light (B) turns out to be red (R) and green (G). Therefore, a white light source is obtained.
However, as shown in FIG. 3, since the color conversion layer 30 is interposed between the bottom electrode 13 and the substrate 11, the flatness of the subsequent organic layer 15 is adversely affected and so is the lifetime of the organic layer 15.
Moreover, one modification of the structure shown in FIG. 3 is to form the color conversion layer 30 on the organic layer 15. In other words, the organic layer 15 is formed prior to the formation of the color conversion layer 30. As a result, the organic layer 15 may be damaged due to the high temperature as the color conversion layer 30 is formed by evaporation, leading poor full-color display quality.
Therefore, in view of the aforementioned problems, there is need in providing a white light emitting organic EL device and a method for fabricating such An organic EL device, resulting to simplify the fabrication processing and achieve longer lifetime as well as better full-color display quality of the device.
Therefore, it is the primary object of the present invention to provide a white light emitting organic electro-luminescent device and a method for fabricating such an organic EL device, employing a simplified fabricating processing to realize a white light source with better full-color quality and longer lifetime of the device.
It is another object of the present invention to provide a white light emitting organic electro-luminescent device and a method for fabricating such an organic EL device, employing a color conversion layer disposed on the inner surface of a scaling cap layer, resulting to simplify the fabrication processing and prevent the color conversion layer from being affected when an organic layer is formed, and vice versa.
It is still another object of the present invention to provide a white light emitting organic electro-luminescent device and a method for fabricating such an organic EL device, employing a transparent sealing cap layer such that the device emits lights of different colors on both sides of the device.
In order to achieve the foregoing objects, the present invention provides a white light emitting organic electro-luminescent device, comprising: a substrate; a bottom electrode formed on a top surface of said substrate; at least an organic layer comprising an organic emitting layer formed on a surface of said bottom electrode to emit a first-color light; at least a transparent opposed electrode formed on a surface of said organic layer; and a sealing cap layer covering said organic layer and said transparent opposed electrode, wherein a color conversion layer is disposed on an inner surface of said sealing cap layer for converting a percentage of said first-color light into a second-color light and a third-color light.
The present invention further provides a method for fabricating a white light emitting organic electro-luminescent device, comprising steps of: providing a substrate; forming a bottom electrode on a top surface of said substrate; forming an organic layer on a surface of said bottom electrode to emit a first-color light; forming an opposed electrode on a surface of said organic layer; and forming a color conversion layer for converting a percentage of said first-color light into a second-color light and a third-color light; wherein said sealing cap layer and said color conversion layer are fixedly disposed on said surface of said substrate, while said organic layer and said opposed electrode are enclosed in said sealing cap layer.